In multiplexers for analog data acquistion systems, a plurality of inputs, such as thermocouples, are scanned by selectively connecting their input lines to a signal bus. The connection is made through a filter having resistors in the input lines and a shunt capacitor for removing normal mode noise. A signal bus connects the selected input through an amplifier to an analog-to-digital converter, and the digitized result is made available for further processing. Relay contacts in the input lines provide the connection of the selected input to the bus. In the past the relays have usually been of the electromechanical type. With such relays common mode noise pickup on the inputs has not presented a problem because of the high resistance between the contacts of electromechanical relays when they are in the open state. Also, the capacity between the open contacts is typically high.
It has become important for a number of reasons to use solid state relays in place of the electromechanical units for this type of service. These switches exhibit poor performance under transient common mode voltages due to self-commutation. In other words, the devices will appear to have a low open circuit impedance in the presence of common mode transients. Also, a higher capacity between the open contacts is experienced. Those conditions lead to a degradation of the common mode noise rejection due to the tendency for common mode noise to charge the capacity in the input circuits as a result of leakage through the open contacts of the solid state relays.
It is an object of this invention to provide a method and apparatus for rejecting both common mode and normal mode noise in analog signal multiplexing circuits which use solid state relays or any other type of relay which exhibits a high coupling capacity and insufficiently high open circuit impedance to transients.